I have owned both plus many other scopes in a wide range of prices. These are two very different scopes! We are all different and you will get different answers… Also, I like nice glass and very happy paying for quality…
NX8 - I owned the 4-32. I have read on the forums that the 4-32 is better optically than the 2.5-20, but I can’t speak to that. I think that the NX8 really only has its size/weight/parallax combination going for it. I would consider it a hunting scope more than a bench scope. That being said, I would happily spend way less money and be happy with something much cheaper as a hunting scope. This is a scope I don’t feel was anywhere near worth the money I paid, approximately $1800. The size does require optical compromises, and I think it shows.
ATACR - I owned the 7-35, so it may be different than the model you may want. I think it is one of the best scopes I have ever owned. The optical quality was top notch. It agrees with my eyes so well. I was able to dial the diopter in better with this scope than any other, so I like that it is not fast focus. The low light was one place where it really really shined for me. I paid $2700 for mine. While I think it was amazing, I don’t think it was worth that either. I was happy to buy two Athlon Cronus scopes for less, while giving up some, but not enough not to be a problem. ATACR was still better.
In that price range and sticking to FFP comparables… The Vortex G3 is amazing and as good or better than the ATACR, but I paid $2000 for it. I wish I had been able to compare them at the same time. I want to try the Theos and the Zeiss S3 6-36 as well, as they are both a touch cheaper and consensus is they are as good as well. I have not used either of those two though. I have owned a Schmidt and Bender as well for over $3k, and I thought it was not worth the money at all.
All these scopes I like, are much bigger in size than the NX8, so they are very different uses for me.